The present invention relates to a fine-line pattern forming method for use in a manufacturing process for a semiconductor IC device and the like, and a material for forming a pattern used in the pattern forming method.
In the manufacture of ICs, LSIs and the like, a pattern is conventionally formed through photolithography using UV, in which a light source with a shorter wavelength has become mainly used in accordance with refinement of a semiconductor device. Recently, a surface imaging process using dry development has been developed in order to increase the depth of focus and improve practical resolution in using a light source with a shorter wavelength.
As an example of the surface imaging process, U.S. Pat. No. 5,278,029 discloses a method in which, after selectively forming a polysiloxane film on the surface of a resist film of a resist material which can generate an acid through exposure, the resist film is dry etched by using the polysiloxane film as a mask, so as to form a resist pattern. Now, this conventional method of forming the resist pattern will be described with reference to FIGS. 5(a) through 5(d).
In this method, a copolymer of 1,2,3,4-tetrahydronaphthyridinenimino-p-styrene sulfonate (NISS) and methyl methacrylate (MMA) is used as the resist material for generating an acid through exposure.
First, as is shown in FIG. 5(a), a resist film 401, which generates an acid through. exposure, coated on a semiconductor substrate 400 is irradiated with a KrF excimer laser 404 by using a mask 403, and thus, the acid is generated in an exposed area 401a of the resist film 401. Owing to this acid, the exposed area 401a is changed to be hydrophilic, so that water in air can be easily adsorbed by the exposed area 401a. As a result, a thin water absorbing layer 405 is formed in the vicinity of the surface of the exposed area 401a as is shown in FIG. 5(b).
Next, when an alkoxysilane gas 406 is introduced onto the surface of the resist film 401, the acid generated on the surface of the exposed area 401a works as a catalyst, so that alkoxysilane is hydrolyzed and dehydrated. As a result, an oxide film 407 is formed on the surface of the exposed area 401a as is shown in FIG. 5(c). Subsequently, when the resist film 401 is dry etched by RIE using O.sub.2 plasma 408 by using the oxide film 407 as a mask, a fine-line resist pattern 409 is formed as is shown in FIG. 5(d).
This pattern forming method thus adopts a negative type lithography process for forming a resist pattern in an exposed area, in which the acid generated in the exposed area of the resist film is used as the catalyst for selectively forming the oxide film in the exposed area and the oxide film is used as a mask in the dry etching for forming the resist pattern.
The negative type lithography process has the following problems in, for example, forming a contact hole for connecting multilayered interconnections of an IC:
First, usage of a mask generally adopted in pattern exposure can cause the following problem: In the lithography for forming a contact hole, the aperture ratio of the mask is very high when the negative lithography process is used. Specifically, while a light shielding film against exposing light is formed merely in a portion corresponding to the contact hole on the mask, the light shielding film is removed and quartz of the mask substrate is bare in the other portion excluding the contact hole in order to transmit the exposing light. Since the area occupied by all the contact holes in the entire area of a semiconductor chip is generally very small, the proportion of the area occupied by the bare quartz to the area of the light shielding film on the mask becomes high, namely, the aperture ratio of the mask becomes high.
When the aperture ratio of the mask is high, the effect of ambient dusts is increased. Specifically, dusts adhered to the light shielding film on the mask scarcely affect the process, but those adhered to the transparent portion of the mask change this portion into a light shielding portion. When the exposure is effected by using the mask to which dusts are thus adhered, a pattern defect is caused in the portion to which the dusts are adhered. In this manner, since the aperture ratio of the mask is high in the negative type lithography process, the process can be easily affected by dusts, resulting in easily decreasing the yield.
Secondly, in the lithography process for forming a contact hole, a half-tone type mask can be used for the purpose of increasing the depth of focus. However, the effect of increasing the depth of focus can be attained merely in a positive type lithography but cannot be attained in the negative type lithography. Accordingly, in the formation of a contact hole, the depth of focus is smaller in the negative type process than in the positive type process.
The occurrence of these first and second problems are not limited to the formation of a contact hole, but can be caused in the cases where a mask having a larger area of the transparent portion is used and where the depth of focus is desired to be increased.